This invention relates to digital to analog converters and in particular to bipolar converters having high accuracy at the zero crossover level.
Digital to analog converters are designed to reconstruct complex and dynamic analog wave forms, particularly audio or acoustic signals, from digital data. When these signals are bipolar, i.e., operating both positively and negatively with respect to a zero crossover, the complexity required within the converter is greatly increased, usually with a loss in accuracy. In such signals, power occasionally peaks well above the RMS power level. There must, therefore, be a very large dynamic range to handle the bursts of higher amplitudes, as well as the ability to handle the wide range of frequency interest. Yet, accurately recreating such information from digital data requires exceptional small signal performance to handle the information near zero level or crossover without crossover distortion.
In appreciation of this problem, digital to analog converters have been developed which separate the digital input signal into a polarity digit and a number of digits representing the absolute magnitude of the digital number, i.e., the magnitude excluding the sign. When this is done, a unipolar digital to analog network can be used to convert the digital signal to its analog equivalent, and a switching or inverting/non-inverting amplifier can be used to restore the polarity to the signal once it has been converted, i.e., depending upon the polarity of the input signal, the output of the unipolar digital to analog network is either inverted or non-inverted. Although this circuit is able to improve the linearity of small signals in the center region of the full scale range, it requires a very accurate and costly inverting amplifier, and therefore tends to remain a limited solution.
Another alternative has been to utilize two digital to analog networks, one of which operates from a positive reference source and the other of which operates from a negative reference source. Depending upon the polarity of the input signal as determined by the sign digit, only one of the digital to analog networks is switched on, with its output being connected to a suitable operational amplifier. In this case only one or the other of the digital to analog converters is used at any given time, again to convert the absolute magnitude of the digital data, i.e., absent the polarity information. This circuit, however, requires that a positively operating digital to analog network be matched with a negatively operating digital to analog network. Since these two digital to analog networks are differently configured, it becomes increasingly difficult to find such a matched pair.